1. Field of the Invention
This invention relates to methods and apparatus used in railway systems for the detection of a collision candidate, such as a forward or rearward approaching locomotive or other track occupying vehicle, and more particularly to the automated invocation of collision avoidance measures.
2. Description of Prior Art
There are occasional collisions of trains that occur even though most locomotives are equipped with voice communication systems that should enable the engineers to detect collision candidates in time to initiate collision avoidance measures. There have been efforts to provide methods and apparatus that will detect and automatically avoid train collisions. The following patents represent proposed solutions to the long standing problem of vehicular collisions, some of them catastrophic, including collisions between locomotives.
2.1 Collision Avoidance Patents
U.S. Pat. No. 2,762,913 discloses a railway train proximity warning system that uses a transmitter and a receiver at each end of a train for sending coded signals, with control means to render each of the receivers inoperative during periods of radiation from the adjacent transmitter.
U.S. Pat. No. 2,783,369 discloses a radio transmitting and receiving signal system for use in a railway system to minimize accidents, the exact location of a train to be determined because the signal will vary linearly with the distance traveled and also indicate by its frequency the exact location of the train. This patent does not disclose any means for automatically avoiding collision and is believed to lack accuracy or relativity.
U.S. Pat. No. 3,365,572 discloses an automatic collision prevention, alarm and control system for use by trains that utilizes continuous beam lasers at each end of the train that are projected ahead and behind the train. Photo electric cell detectors are used to provide an audible or visual warning to the engineer and, at the same time, automatic control circuits may be operated to effect an emergency brake application or the automatic closing of the throttle. This system is not believed to be able to differentiate between locomotives, trains and railway maintenance-of-way (MOW) equipment, but rather, can give false indications caused by animals or automobiles at grade crossings.
U.S. Pat. No. 4,473,787 discloses a system for maintaining the spacing of trains or other track bound vehicles. It utilizes a light emitter, operating with modulated light and radiating uniformly and a light receiver to reduce the speed of the vehicle with increasingly received light intensity.
U.S. Pat. No. 4,701,760 discloses a method for monitoring vehicles from a central station by obtaining the approximate coordinate of the vehicle from signals transmitted by stations of the world omega network, using vehicle carried receivers. The approximate coordinates are corrected by reception and processing means connected to a fixed radiogoniometry beacons. Processing means connected to the vehicle-carried receiver supply the real coordinates of the vehicle to a vehicle-carried transmitter. The actual coordinates are transmitted in actual form to a receiver at a central station. Alarms on board the vehicle can be transmitted to the station.
U.S. Pat. No. 4,897,661 discloses a method and apparatus for determining the position of a vehicle with a system utilizing a transponder in each vehicle that transmits a signal responsive to an interrogation signal to a ground station through two or more satellites. The position of the vehicle is determined from the propagation time differences of the response signals received from the satellite.
U.S. Pat. No. 5,068,654 discloses a collision avoidance system that addresses the problem involved where there is a large number of vehicles in the same general area. Each vehicle is equipped with a collision avoidance transponder for transmitting and receiving data from the other vehicles. A central reference time signal generator is provided in a neutral position in order to transmit a periodic reference timing signal for reception at the transponders. Upon the transponder receiving the reference timing signal, it subsequently transmits information data relating to that vehicle for reception by the other vehicles. Each transponder is allocated a specific time slot or period which is unique to that vehicle for transmitting the information data.
U.S. Pat. No. 5,210,534 discloses an encoding method for anti-collision systems for sea navigation. Here, a transmitter aboard a ship transmits its geographical coordinates, speed and course, as well as an identification code.
U.S. Pat. No. 5,307,074 is another collision avoidance system for sea navigation. A transmitter aboard ship transmits its geographic coordinates, course and speed and a display exposes similar data from other ships. The received data are displayed, mostly in the form of symbols on the panoramic screen of the display device.
2.2 The GPS System
In the early 1980s , the Global Positioning System (GPS) satellite system began to come on line. GPS will eventually comprise 24 satellites in orbits approximately 11,000 miles above the earth's surface inclined 50 degrees from the equator. The satellites are not in a constant position, but have a 12 hour orbit. At any point on the earth a ground base receiver normally can receive signals from four GPS satellites. The satellites are tracked from ground stations by the U.S. Air Force. A basic explanation of GPS and its use in surveying is given in Hurn, GPS, A Guide to the Next Utility, Trimble Navigation 1989, incorporated herein by reference.
(a) PRN Code Information Signals. The GPS satellites each transmit signals containing groups of information that enable distance measurements to be made. There is transmission of a pseudo-random number (PRN) code from each satellite to a GPS receiver.
The PRN code is a very faint signal which hardly registers above the earth's natural background noise, yet can still be received by an antenna only inches in size. Decoding of these signals is accomplished in known fashion by sampling the PRN code and comparing the code with previously received signals over time, permitting the PRN code to be picked out of the earth's background noise.
The PRN code includes the time (as measured by an atomic clock aboard the satellite) at which the signal left the satellite. Over time, the signals include information about the satellite's current orbit in space as well as corrections for known errors in the satellite's clock (uploaded to the satellite periodically by the Department of Defense).
Two types of services produce signals from the GPS satellites. First, the Precise Positioning Service (PPS) produces for the military the most accurate dynamic positioning possible with GPS, based on the dual frequency Precise or Protected code known as the P-code. Users must have an encryption code to access the P-code which is not available to the public. Standard positioning service (SPS) produces the publicly accessible civilian positioning accuracy obtained by using the single frequency "Clear Acquisition" (C/A) code. The Department of Defense (DoD) has the ability to degrade the accuracy of the C/A code using "selective availability" (S/A) or artificially creating clock errors to prevent hostile military forces (e.g., jet aircraft) from navigating using the C/A code.
(b) Computation of Position Fixes Using GPS Signal Data. An earth-based receiver receives the PRN code and records the receive time as measured by the receiver's clock. Taking relative clock offsets into account, the difference between a signal's departure time and arrival time is the total travel time. The distance from the GPS satellite to the receiver's position is
then approximated by multiplying the speed of light times the total travel time (refraction-and other errors must be taken into account as discussed in more detail below).
A minimum of three satellite signals are needed to determine position using GPS satellites. If all that is known is that the position in question is 11,000 miles from a first satellite the position could be anywhere on a sphere centered on that satellite and having an 11,000 mile radius. If two satellite distances are known, the position could be anywhere on a circle where the two satellite-centered spheres intersect. If three satellite distances are known, the position can only be at one of two points at which all three spheres intersect. For a position known to be on earth, one of these points will usually be a ridiculous answer (e.g., somewhere in space), so three satellites are enough to pinpoint a location.
(c) Atomic-Clock Timing of Satellite Signals. Because the speed of light is so fast, it only takes about 6/100ths of a second for a GPS satellite signal to travel to earth. To avoid satellite-distance errors (and therefore position-fix errors) arising from clock errors, very accurate clocks must therefore be used in both the satellite and receiver. GPS satellites use "atomic" clocks, that is, clocks driven by an atomic-standard oscillator, e.g., a cesium- or rubidium-standard oscillator. (Atomic clocks do not necessarily run on atomic energy, but use the oscillations of an atom as their "metronome" to keep extremely accurate time.)
Many satellite receivers use quartz-crystal oscillator clocks of lower accuracy than atomic clocks, whose cost is presently quite high. Collection of PRN code data from a fourth satellite can be made to compute time of arrival (by simultaneously solving four satellite distance equations with four unknowns) using only the satellites'on-board atomic clocks to eliminate the need for an atomic clock on the ground. Even with atomic clocks aboard the satellites, however, some error in timing occurs. Each satellite uses four atomic clocks which are additionally synchronized by the Colorado Springs Falcon Air Force Station with a cluster of clocks, or master time base to keep absolute time. Each satellite's four clocks or master oscillators have individual characteristic drifts. The drift errors are kept defined based on a master time base ("GPS time"). The satellites transmit the timing drift errors as part of the PRN code.